Since the introduction of highly resilient, pressure-energized metallic sealing rings, such as that according to U.S. Pat. No. 3,797,836, in the early 1970's, temperatures and pressures in turbine engines, where such rings may be employed, have increased substantially. Increased operating temperatures and pressures have caused the magnitude of the displacements between cooperating members of sealing surfaces to correspondingly increase. These cooperating sealing surfaces must seal together, under all operating conditions, to contain, for example, working gases at temperatures up to 1800.degree. F. and cooling air at over 1200.degree. F.
One way to provide effective sealing for the increasingly large and variable displacement between cooperating members of sealing surfaces is by providing multiple convolutions between cantilever sealing members at each end of a sealing ring, as shown, for example, in U.S. Pat. No. 4,121,843. As displacement continued to increase, a further response was desired. This came in the form of multiple-ply sealing ring construction, in which the thinner plies, capable of containing the fluid under pressure, when used in layers, provided the capacity for up to two or more times the deflection at a given stress level compared to single-ply seals. U.S. Pat. Nos. 5,249,814 and 5,716,052 describe sealing rings using multiple-ply technology.
Multiple-ply sealing rings, such as that described in U.S. Pat No. 5,249,814, can be expensive because, for example, a seal may be required to have its edges welded together to prevent the ingress of pressurized media between the plies, where such ingress would cause overstressing of the material of the ply at the greatest distance from the higher pressure source. Seals such as that disclosed in U.S. Pat. No. 5,716,052 overcame that cost barrier associated with use of multiple plies by folding over the edges at each end of the seal section. For illustration purposes, a cross-section of a seal 1000 according to U.S. Pat. No. 5,716,052, is depicted in FIG. 10. By folding over the edges 1002, 1004, 1006, and 1008 at each end of the seal 1000, so that the openings to the interstitial space 1010 between the plies 1012 and 1014 were exposed to the lower pressure zone instead of the higher pressure medium being sealed, the seal 1000 depicted in FIG. 10 prevented the ingress of pressurized media between the plies.
However, because each of the bends at the folded ends of seals, such as a seal depicted in FIG. 10, have two thicknesses of seal material for every ply of the seal, such seals had several disadvantages. Among the disadvantages were stiffness at the termination regions (ie., the folded ends in the region of the seal's sealing lines), where flexibility is needed; inactivity at the tightly folded edges of the seal, providing no flexibility to accommodate longitudinal axial displacement; and the consumption of critically necessary axial space.
A seal consisting of three plies, such as are described in U.S. Pat. No. 5,716,052 has six layers of material and three folds at each end.